Malaysian Journal of Civil Engineering 25(2): 128

Malaysian Journal of Civil Engineering 25(2):128-153(2013)
ULTIMATE STRENGTH ASSESSMENT FOR FIXED STEEL
OFFSHORE PLATFORM
Narayanan Sambu Potty1* & Ahmad Fawwaz Ahmad Sohaimi1
1
Department of Civil Engineering, Universiti Teknologi PETRONAS, 31750 Tronoh,
Perak Darul Ridzuan, Malaysia
*Corresponding Author: [email protected]
Abstract: Currently, more than 80% of Malaysia’s offshore platforms are aged 30-40 years
which is beyond the design life of 25 years. Structural assessments are needed to gauge the
platforms for the extended use. The two common methods widely used are the simplified
ultimate strength analysis and static pushover analysis. Simplified ultimate strength is attained
when any of member, joint, pile steel strength and pile soil bearing capacity reaches its ultimate
capacity. This is the platform’s ultimate strength. Static pushover analysis generally concentrates
on RSR (Reserve Strength Ratio) and RRF (Reserve Resistance Factor) for the ultimate strength.
This report summarizes a study of the ultimate strength of jacket platforms designed using API
RP2A-WSD 21st Edition (2000) using SACS software with the module for Full Plastic Collapse
Analysis. Two types of analyses have been carried out. First the ultimate strength of jacket
platform with different number of legs is determined and in the second part the collapse load of
platforms for different bracing configuration is studied. The non-linear pushover analysis is done
by programming the software to analyze the structure with a set of incremental load until the
structure collapses. The non-linear analysis module will distribute the load to alternative load
paths available within the jacket framework until the structure collapse or have excessive
deformation. In order to cater to uncertainties and distribution of data, several criteria of platform
site location, age of service, type of platform, number of legs and other critical-related
characteristics were considered. From the first phase of study, it is seen that \ a platform with
more legs has higher ultimate strength compared to less number of legs. Hence a bigger jacket
platform with eight legs is stiffer than smaller platform. The bracing configuration study shows
that the X-bracing contributes highest rigidity to the whole platform by retaining the platform
until the highest load compared to other configurations.
Keywords: Aged platform, Collapse analysis, Reserve strength, Load factors
1.0
Introduction
Offshore structures are used for oil and gas extraction from under the seabed. It provides
a safe, dry working environment for the equipment and personnel who operate the
platform. Offshore structures are of two categories namely fixed platform and floating
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means
without the written permission of Faculty of Civil Engineering, Universiti Teknologi Malaysia
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
129
platform. Examples of fixed platforms are steel-jacket platform, jack-up and compliant
tower while examples of floating platform are spar, semi-submersible and FPSO. Jacket
platforms have a design life in the range of 25-30 years. But many platforms in Malaysia
are about 30-40years old. Some of the very early platforms are still in service. Over the
last 10 years or so, various structural integrity assessments have been carried out on the
platforms to gauge its safety and usability beyond the design life. Assessment of
structural integrity can be done qualitatively, semi-qualitatively or quantitatively. The
first step for all these is knowledge on basic information (as-built) on platforms, which
was reported for Malaysian platforms by Akram and Potty (2011a). Inspections
including underwater inspections are also carried out (Akram and Potty, 2011b).
Assessment of Malaysian platforms using semi-quantitative method has been reported in
Potty et al. (2012). Not much work on quantitative assesment of Malaysian platforms
have been reported. Data collection for such methods include information on resistance
parameters as reported in Idrus et al. (2011a; 2011b) and environmental load as reported
in Idrus et al. (2011c). Quantitative structural assessments can be carried using
reliability methods as reported in Cossa et al. (2011a, 2011b, 2012a, 2012b). Alternately
platforms can be analyzed using pushover analysis. The latest metocean data and SACS
(EDI, 2006) input file (model) for the jacket platform are required for the analysis.
1.1
Simplified Ultimate Strength Analysis
Assessment for an aged structure involves documentation of design basis, design level
analysis and ultimate strength analysis. The design basis includes key parameters
including design life, methodology, standards and codes, design parameters for wind,
wave, current, seismic, boat impact etc. An analysis is done mainly to determine the
total strength of a structure. Checking for the ultimate strength is done with respect to
API RP2A-WSD (2000). Excessive deformation or resistances to total collapse are
measures to judge the structural integrity. The structure strength is determined from
static pushover analysis and cyclic loading for severe storm condition. API RP2A-LRFD
(1993) developed based on reliability based calibration, checks the platform for
combined action of extreme wave (storm condition), current and wind that consider the
joint probability off-occurrence. The wave forces were computed using the drag and
inertia coefficients (API RP2A-LRFD, 1993): For smooth surface Cd = 0.65, Cm = 1.60
and for rough surface Cd = 1.05, Cm = 1.20. The code gives equations for checking the
cylindrical members under tension, compression, bending, shear and combined loads.
Members under combined axial tension and bending should be designed to satisfy
equation (1).


  f t   f by ²   f bz ² ½
1  cos 
1.0

b Fbn 
 2t Fy 
(1)
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Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
Where,
fby = bending stress about member y-axis (in-plane)
fbz = bending stress about member z-axis (out-plane)
Fbn = nominal bending
Fy = yield strengths
Ft = axial tensile stress
Φt = resistance factor for axial tensile strength (= 0.95)
Φb = resistance factor for bending strength (= 0.95)
Members under combined axial compression and bending should be designed to satisfy
equation (2).
1
2
22



 




 f c   1   C my f by    C mz f bz   


 
   1.0
 c Fcn    b Fbn   

fe  
fe 


1 

 1 
   c Fez    




F

 

c ey  
 

(2)
And
   f    f  

   f c
1  cos 

 2 c Fxe 
2
by
bz
b Fbn 
1
2 2
 1 .0
Fc  c Fxc
(3)
(4)
Where,
Cmy = reduction factor corresponding to the member y-axis
Cmz = reduction factor corresponding to the member z-axis
Fey = euler buckling strength corresponding to the member y-axis
Fez = euler buckling strength corresponding to the member z-axis
Fey 
Fy
 
2
(5)
y
Fez 
Fz
 z 2
(6)
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
131
λ = column slenderness parameter for member about the respective axes
Fcn = nominal axial compressive strength
Fc = axial compressive stress due to factored load
Φc = resistance factor for axial compressive strength, 0.85
The equations for strength checks of tubular joints are also given in API RP2A-LRFD
(1993).
For assessment of existing platforms, the criteria are dependent on the category of the
platform, which considers the life safety, and the consequences of failure. Krieger, et al.
(1994) has recommended two factors for ultimate strength checks for existing platforms
namely:


Ultimate to Linear Ratio (ULR)
Reserve Strength Ratio (RSR)
ULR is the ratio of the ultimate resistance load to that causing a unity check of 1.0 in the
original design and RSR is the ratio of the ultimate strength load to the storm condition
(100-year) design load. For manned platforms with or without significant environmental
impact, a ULR of 1.8 and RSR of 1.6 are recommended, while for platforms of
minimum consequence a ULR of 1.6 and RSR of 0.8 are recommended.
Simplified Ultimate Strength (SUS) is generally estimated based on the smallest of the
four base shear values obtained when the first of the following component classes reach
its ultimate capacity namely joints, members, pile steel strength, and pile soil bearing
capacity, The platform base shear values that satisfy each of these conditions are
determined from a linear analysis by using respective API RP2A-LRFD equations with
the load and resistance.
In simplified approach, a linear static global analysis of the structure is performed for
forces due to the combined action of gravity loads and extreme wave loads (100-year
return period) and associated current and wind effects. The structure is loaded with
series of monotonically increasing environmental load conditions from all directions of
interest. Member and joint forces are obtained from the analysis and for each load
condition the strength checks are made for the members, joint and etc using API RP2ALRFD. The load is increased after each stage until any component of the structure fails
or reaches its ultimate strength. The platform attains ultimate strength when any member
or joints reach its ultimate capacity. The first member/joint failure is obtained and the
load factor corresponding to this is calculated as ratio of the base shears corresponding
to the first member failure and the 100-year environmental load. The analysis is further
performed by removing the failed member from the model, if alternative load paths are
available to bypass a failed member. The analysis is terminated when there is no
alternative load path or deformation of the structure exceeds the limit from functional
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considerations. The reserve strength ratio is then calculated as the ratio of the base
shears corresponding to collapse load and first member failure. Full ultimate strength
analysis using non-linearity can be resorted to if the simplified ultimate strength analysis
does not meet the requirements for requalification.
API RP2A-LRFD recommends using linear wave theory and Morrison equation to
calculate the wave and current loads on the structure. Yield stress of steel is taken as per
design basis requirement. The base shear for first member failure is obtained from each
attack angle to get the factor of first member failure, the factor for collapse load and the
reserve strength ratio. The output from analysis is categorized as (1) Lateral load for
100-year storm condition, (2) First member failure load, Pmf, (3) Factor for first member
failure, (4) Collapse load, Pu, (5) Factor for collapse load, (6) Deformation
corresponding to Pmf, (7) Deformation corresponding to Pu, and (8) Reserve strength
ratio. The factors are calculated as follows:
(7)
(8)
(9)
Another approach proposed by Vannan et al. (1994) where a linear static in place
analysis is done by increasing environmental loading until first member or other
component failure occurs. Unity check reported above 1.0 is allocated as the ultimate
strength of the structure. Other simplified methods introduced by Bea and Mortazavi
(1995) provide reasonable estimates of platform load capacity relative to the results
obtained from the detailed static pushover analysis.
1.2
Static Pushover Analysis
Research on the response of jacket structure to extreme condition (100-year return
period storm wave) requires the estimation of the ultimate strength of the framed
structure as well as its reserve capacity . An elastic frame analysis is performed,
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
133
typically with the elements assumed to be rigidly connected. API RP 2A-WSD 21st
Edition, Section 17.0 recommends for the assessment of existing platforms a sequence
of analysis from screening, through design level to ultimate strength assessment to
demonstrate the structural adequacy. At the ultimate strength level, a platform may be
assessed using inelastic, static pushover analysis.
Lloyd and Clawson (1984) discusses the sources of reserve and residual strength of
‘frame behaviour’. Marshall (1979) studied behaviour of elastic element and ultimate
strength of the system. Marshall and Bea (1976) demonstrated the reserve safety factor
and Kallaby and Millman (1975) studied the inelastic energy absorption capacity of the
Maui A platform under earthquake loading. Recent investigation shows that static
pushover analysis generally suffices to demonstrate a structure’s resistance to the cyclic
loading of the full storm.
Trends for lighter, liftable jackets and new concepts for deepwater have provided
additional impetus for such studies. Fewer members in the splash zone may increase the
risk to topsides safety in the event of impact, and the deletion of members with the low
elastic utilizations to save weight reduces the capacity for redistribution along the
alternative load paths. Comparative calculation of reserve capacity for different
structural configurations can help ensure that levels of reserve strength and safety
embodied within the older designs are maintained. Therefore there is a requirement to
develop an understanding and the corresponding analytical tools to predict system
reserves beyond individual component failure capacities, in order to demonstrate
integrity in the event of such extreme loading scenarios.
Reserve strength is defined as the ability of the structure to sustain loads in excess of the
design value. RSR (Reserve Strength Ratio) introduced by Titus and Banon (1988) and
RRF (Reserve Resistance Factor) introduced by Lyod and Clawson (1984) are defined
below:
RSR 
RRF 
UltimatePlatform Re sis tan ce
DesignLoad
EnvironmentalLoadatCollapse
DesignEnvironmentalLoad
(10)
(11)
Fixed offshore structure spread the load through a network of paths. As a result the
failure of a single member does not necessarily lead to catastrophic structural collapse.
The redundancy in the structure is measured in two ways, namely (1) redundancy factor
(RF) and (2) the damaged strength rating (DSR). These measurements are load case
dependent and any structure may exhibit very different redundancy properties for
different loading directions.
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Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
Reserve strength is evaluated by applying the maximum loading from the extreme event
and then performing the ‘pushover’ analysis. For an extreme storm, the environmental
loading is cyclic, imposed in an underlying dominant direction. The maximum wave is
unlikely to be an isolated event, but will be a peak in series of extreme loads. The
possibility of cyclic degradation of components which have failed, or approaching
failure even though overall structure resistance may remain adequate, therefore needs to
be considered.
Static pushover analysis is the application of a single load, applied to any specific
location which is incremented in steps until collapse while cyclic analysis is a ‘storm
load’ sequence of particular amplitude applied to the structure. Shakedown effects were
studied using non-linear FE analysis at SINTEF (Hellan et al., 1991, 1993, 1994) for the
provision of low cycle-high stress fatigue. These studies on North Sea Jackets,
recommend that an extreme event static analysis generally suffices to demonstrate
structure’s resistance to the cyclic loading of a full storm. Research was also carried out
supported by Shell (Stewart et al., 1993, 1998; Stewart and Tromans, 1993; Eberg et al.,
1993; and Hellan et al., 1991, 1993, 1994). Under the increased loading, the structure
converts into elasto-plastic range, yielding occurs thereby reducing the stiffness and
introducing permanent plastic deformations. Under cyclic load, the yield repeats and
result in three different forms of response namely Low cycle fatigue, Incremental
collapse and Shakedown.
2.0
Methodology
The scope of work involves the following studies on aged platforms:
(1) Full Plastic Collapse Analysis of 3, 4, and 8 Legged Platforms (A, B and C
respectively) and evaluation of the factor for first member failure, collapse load
factor and RSR
(2) Bracing configuration study for platform A and determination of collapse load and
RSR.
The methodology is described below.
2.1
Full Plastic Collapse Analysis of 3, 4, and 8 Legged Platforms
SACS modelling for platforms A, B and C commenced by adjusting the original model
with the site visit findings and latest drawing. The dead and live load of the SACS
model were retained as per the design basis. Minor adjustments were made to the model
in terms of the latest metocean data for the area. Latest data of maximum wave height
(Hmax), associate period (Tass), wind speed, current speed and tidal height, HAT and
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
135
LAT were used. The environmental loading impact on the platform considers eight (8)
directions. The storm condition is applied to the platform according to the metocean data
as the maximum load acting on the structure. Stokes’s 5th order theory defined in API
RP2A-WSD is used for wave / current loading computation. For the purpose of analysis,
eight (8) models were created for platform A and C while twelve (12) models were
prepared for platform B which is a tripod and required 12 directions to be covered.
The SACS Collapse module is a non-linear finite element analysis system for structures.
It can solve for the geometric and material non - linearities and determine the ultimate
load capacity by using large deflection, iterative direct stiffness solution technique. The
members are divided into several sub-segments along the length and sub-areas to define
the cross section. The method allows for gradual plasticification along the member
length. Tubular connection flexibility, capacity and failure are revised empirically
during the analysis. The linear analysis model is modified to be suitable for collapse
analysis. The model is designed to cater for the storm condition wave/current in order to
get the strength of the structure under maximum loading criteria, and then the model
undergoes the SACS COLLAPSE analysis. The load sequence and load increment in
collapse input file is prepared based on the design basis. The other properties in the
collapse input file are retained as per the default design. The SACS model was modified
to apply wave and current loads for different directions. For a tripod jacket model, 12
models are required to cater for the 12 attack angles as defined in the metocean data.
The four and eight legged platform models were modified to cater for only eight
directions as defined in their metocean data. The main issue is to analyze the jackets for
all the directions of loading and to determine the direction having the highest collapse
load. A series of incremental load defined in collapse input file will generate collapse
load by utilizing the module of FULL PLASTIC COLLAPSE ANALYSIS in the SACS.
Upon completing the analysis, the output available are (1) Base shear and overturning
moment, (2) Basic load case summary, (3) Load combination summary, (4) First
member failure load, (5) Collapse load. The factor for first member failure and reserve
strength ratio based on base shear and collapse load are determined. Collapse view
module is used to view the platform collapse mode and properties. The data is also used
to determine the structure ultimate strength and corresponding wind/ wave direction.
Figure 1 shows the flow chart of full plastic collapse analysis.
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Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
Figure 1: Flow chart of full plastic collapse analysis (Fawaz, 2010)
2.2
Bracing Configuration Study
This involves collapse analysis of a selected jacket with varying of bracing schemes.
Some common bracing types are X bracing, Y bracing, Single diagonal bracing, K
bracing, Inverted K bracing and Diamond bracing (Figure 2). Figure 3 shows the flow
chart for the bracing configuration study.
Figure 2: Bracing framework schemes (Nelson, 2003)
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
137
Figure 3: Flow chart of bracing configuration study (Fawaz, 2010)
The share of the load carried by different bracing schemes differs. The platform A was
remodeled to cater for all types of bracing. Utilizing linear analysis, each bracing design
will undergo stability check in term of UC value of the respective bracing. Allowable
value of UC<1.0 indicate that the new designs are acceptable for the collapse analysis.
For different bracing schemes of platform A, the identical bracing properties of size,
shape and wall thickness were used. New joint was allocated at the critical location
when modelling for X-bracing and K-bracing where the member intersects at the middle.
All bracing models were analyzed and compared. The collapse load of the jacket for
different bracing framework strength was determined. The maximum load for first
member failure was noted for each case. The output obtained from the analysis consists
of (1) base shear and overturning moment, (2) Collapse Solution Summary, (3) Collapse
Load, (4) Maximum deflection and (5) RSR.
3.0
Platform Overview
Platform A is a four pile-through-leg drilling platform installed in 1979 and located in
PMO (Figure 4). One boat landing is on the Platform South face and other two boat
landings are on the Platform West face. The topside comprises of the Upper Deck (EL
+19202), Lower Deck (EL+12192). Details of the structure are in Table 1 and
environmental data are in Table 2. Platform B in SKO is a three pile-through-leg
platform installed in 1977 (Figure 5). The platform supports four numbers of 10.75” Ø
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(27.31 cm) risers. Details of the structure are in Table 1 and environmental data are in
Table 2. Platform C is a eight pile-through-leg drilling platform installed in 1979 in
PMO (Figure 6). The topside comprises of the Upper Deck (EL +21184), Lower Deck
(EL+14021). Details of the structure are in Table 1 and environmental data are in Table
2.
Figure 4: Platform A
Figure 5: Platform B
Figure 6: Platform C
Table 1: Platform description
Structure Function
Installation Date
TAD Rig
Water Depth(MSL)
No. of Piles
Pile
penetration
below
mudline
Number of Conductor
Diameter of conductor
Number of Anode
Number of Boat landing
Number of pipe Caissons
Number of Riser pipes
Number of Riser Guard
Platform A
(PMO)
Drilling Platform
1979
Jack-Up
70.71 m
(209.56 ft)
4
54”Ø,
137.16cm
79.25 m
Platform
(SKO)
1977
70.93 m
(236 ft)
3
30”Ø,
76.20cm
68m
12 nos
24’Ø, (66.96 cm)
136
3
3
2
1
136
1
4
-
B
Platform C
(PMO)
Drilling Platform
1979
67.21 m
(209.56 ft)
8
54”Ø,
137.16cm
109.73m
32
24’Ø, (66.96 cm)
1
1
10
2
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
139
Table 2: Environmental Data for platform design
Wave
Parameter
Direction
(degress)
Max. Height,
Hmax (m)
Assoc. Period,
Tass (s)
Directions
Max. Height,
Hmax (m)
Assoc. Period,
Tass (s)
100-Year Directional Wave (deg)
Platform A (PMO)
0
42.11
90
137.89
180
222.11
270
317.89
6.3
6.3
11.4
7.6
7.6
7.6
5.0
6.3
7.3
7.3
9.3
8.4
8.4
8.4
6.6
7.3
Platform B (SKO)
N
NE
E
SE
S
SW
W
NW
10.0
9.0
5.1
5.1
5.1
6.9
9.0
10.0
9.7
9.4
8.3
8.3
8.3
8.6
9.4
9.7
Platform C (PMO)
Direction
(degress)
Max. Height,
Hmax (m)
Assoc. Period,
Tass (s)
4.0
0
65
90
115
180
245
270
295
5.8
7.3
10.1
8.2
5.8
5.8
5.8
5.8
8.0
8.5
10.0
9.0
8.0
8.0
8.0
8.0
Computer Modelling
The program SACS (Structural Analysis Computer System) used to perform the
analyses described in the study was developed by EDI (Engineering Dynamic Inc). The
full plastic collapse module is used for the purpose of determining the collapse load.
The SACS input files were checked for latest metocean data and latest modification to
the actual structure. Localized wave direction model introduced to design a model caters
only to 1 direction per analysis. In order to determine the platform critical direction, the
effects from each wave attack direction were compared. In the analysis, 4 and 8 legged
platforms were designed for 8 directions of waves whereas the tripod platform was
designed for 12 directions. The collapse input file for the model consisted of series of
incremental load by a defined factor until the structure collapses; when no load paths are
available or deformation exceeds the allowable value.
Different leg arrangement or complexity of the SACS input file require more period for
completion of the analysis. Table 3 gives the duration for modeling and analyzing for
each platform. The table shows that a complex and larger platform required more time
to complete an analysis.
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Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
Table 3: Average duration of analysis
No of leg
3
4
8
5.0
Period of modeling (hr)
1.0
2.0
2.5
Period of analyzing (hr)
0.5
4-6
6-8
Results of SACS Analysis
Two types of analysis were carried out and the results of each are given below:
5.1
Full Plastic Collapse Analysis of Platform with different number of legs
Final values of each analysis for all three (3) platforms which are located in different
block (PMO and SKO) and has different arrangements are compared (Table 4,5, and 6).
Table 4: Platform B (Tripod) Collapse Analysis Results (SKO)
Table 5: Platform A (4 legged) Collapse Analysis Results (PMO)
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
141
Table 6: Platform C (8 legged) Collapse Analysis Results (PMO)
Table 4, 5 and 6 shows the analysis output from the tripod, 4 legged and 8-legged
platforms subject to series of incremental loads predefined in the Full Plastic Collapse
module of SACS. Wave directions in the metocean data (Table 2) for respective
platforms in the PMO and SKO are considered. Lateral load for 100-year storm
condition correspond to the base shear at the base of the structure. The base shear is
computed using the Stokes’s 5th order theory for computing the drag and inertia forces.
The first member failure is the indication of the first member or other component
reaching ultimate capacity within the plasticity zone.
Table 7 Column 4 shows the effect of number of jacket legs on the collapse loads. The
existence of alternative load paths through leg member and bracing provide more
stiffness to the structure. Table 7 Column 4 clearly indicates a relation between the load
needed for structure collapse and the number of legs and direction. Furthermore,
platform C with eight legs needed larger loads for collapse compared to platform A with
4 legs and platform B, tripod type. With more legs, the structure configuration is more
complex, rigid and strong. This is proved by the behaviour of platform C in retaining
larger loads. Column 5 shows the RSR values, which indicates that platform B with
tripod leg had more strength reserve compared to the more redundant structures. The
design was made for three leg jacket structure to have more reserve strength before the
structure reached the critical point of ultimate load.
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Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
Table 7: Full Plastic Collapse Analysis of Different Legged Platform
Platform
B
A
C
Wave
Direction
(deg)
0.00
30.00
60.00
90.00
120.00
150.00
180.00
210.00
240.00
270.00
300.00
330.00
0.00
42.11
90.00
137.89
180.00
222.11
270.00
317.89
0.00
65.00
90.00
115.00
180.00
245.00
270.00
295.00
First Member
Fail Load
(kN)
3298.68
4017.31
3207.17
2678.52
3296.41
3522.15
3299.57
2775.55
3208.95
4134.51
3300.51
2694.31
9230.77
7844.51
12226.07
8140.04
10582.18
8396.79
10746.32
7944.94
30781.52
33858.24
32464.96
34632.22
32551.37
6809.40
30910.52
26443.94
Collapse Load
(kN)
RSR
Factor For
Collapse
4136.79
4331.39
4020.10
3956.14
5142.96
4260.86
3880.84
4262.99
3618.19
4491.81
3736.09
4287.16
9488.49
9174.21
12974.80
9489.52
11465.45
10078.17
11690.44
9018.19
31730.06
34839.44
37317.46
38437.97
35644.39
6809.40
31968.74
32003.96
1.25
1.08
1.25
1.48
1.56
1.21
1.18
1.54
1.13
1.09
1.13
1.59
1.03
1.17
1.06
1.17
1.08
1.20
1.09
1.14
1.03
1.03
1.15
1.11
1.10
1.00
1.03
1.21
2.39
2.60
2.27
2.31
2.91
2.56
2.24
2.56
2.04
2.62
2.11
2.57
4.05
3.35
2.18
2.76
3.17
2.54
5.72
4.10
9.69
5.10
3.00
4.74
9.52
1.56
6.68
7.16
Table 8 and Figure 7 enable identification of the critical attack angle of wave on the
structure.
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
143
Table 8: Wave attack angle for maximum and minimum values of collapse load for platforms
Platform
B
A
C
Minimum value (kN)
Wave attack
Load
angle (deg)
90.00
3956.14
42.11
9174.21
245.00
6809.40
Maximum value (kN)
Wave
attack
Load
angle (deg)
270.00
4491.81
90.00
12974.80
115.00
38437.97
The maximum value of the tabulated data in column 4 for each platform indicates the
strongest direction of the structure and the highest load (base shear) it can retain before
it collapses. The minimum value of the tabulated date in column 4 for each platform
indicates the critical angle to the structure having the minimum load (base shear)
required for the structure to fail. First member failure load indicates the base shear force
at which first member failure is observed.
This study considers the relationship of the wave generated forces in different directions
to the corresponding collapse load. The highest value of the base shear indicates the
collapse load at the angle. For platform B, considering the different directions, the
highest base shear is generated at the angle of 300 degree to the platform. Non-linear
analysis indicates that the minimum load for structure to collapse occurs at the angle of
90 deg while at 270 degree, maximum load is needed. For platform A, the highest
reported base shear is at 90 degree. The platform C metocean data (Table 2) show that
the critical wave force occurs at 90 degree angle but the collapse load analysis (Figure
7) shows that 90 degree direction is not critical (whereas the 250 degree angle is
critical).
Platform with different number of legs showed different behaviour, response and
rigidity when subjected to collapse analysis. Complexity and rigidity are the main
criteria for a platform to have higher ultimate strength.
144
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
Figure 7: Collapse Load VS Wave Direction
Figure 8: Collapse Load factor VS Wave Direction
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
145
Figure 9: RSR VS Wave Direction
5.2
Bracing Configuration Study
The different bracing configurations for platform A used for analysis are (1) X bracing
(Figure 10), (2) Design basis (Figure 11) and (3) Single Diagonal bracing (Figure 12).
Figure 10: X Bracing
Figure 11: Design basis
Figure 12: Single Diagonal Bracing
146
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
The design basis and the single diagonal bracing differ only in the top and bottom panel
where the design basis platform has extra member as shown in Figure 9. The results of
the analysis are presented in Table 9. The performance of the bracing schemes is
assessed by the following plots which compares and differentiates them.
1. Collapse loads Vs Direction (Figure 13)
2. Collapse Load Factor Vs Direction (Figure 14)
3. RSR Vs Direction (Figure 15)
Design Basis
Table 9: Bracing Configuration Study for platform A (4 legged)
First
Wave
Collapse
Factor
Member
Configuration
Direction
Load
RSR
for
Fail Load
(deg)
(kN)
Collapse
(kN)
0.00
9488.49
1.03
4.05
9230.77
42.11
9174.21
1.17
3.35
7844.51
90.00 12226.07 12974.80
1.06
2.18
137.89
9489.52
1.17
2.76
8140.04
180.00 10582.18 11465.45
1.08
3.17
222.11
1.20
2.54
8396.79 10078.17
270.00 10746.32 11690.44
1.09
5.72
317.89
9018.19
1.14
4.10
7944.94
0.00
1.02
4.27
10410.32 10630.12
42.11
9144.72
1.10
3.15
8311.26
90.00
1.32
2.44
11604.26 15280.47
137.89
10042.01
1.19
2.75
8449.42
180.00
1.02
3.02
11397.21 11584.19
222.11
10862.32
1.35
2.58
8049.95
270.00
1.15
5.76
10816.69 12445.34
317.89
9885.51
1.24
4.24
7960.63
0.00
9493.51
1.03
4.06
9215.76
42.11
9029.29
1.15
3.31
7826.34
90.00
1.08
1.89
10424.99 11221.38
137.89
9125.70
1.10
2.66
8296.00
180.00
1.05
3.08
10557.71 11097.71
222.11
10600.58
1.24
2.68
8568.37
270.00
9751.93
1.01
4.79
9683.77
317.89
9527.65
1.20
4.34
7929.77
X-Bracing
Single Diagonal
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
Figure 13: Collapse Load Vs Wave direction
Figure 14: Collapse Load factor Vs Wave direction
147
148
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
Figure 15: RSR Vs Wave direction
From Table 9 and Figure 13, it is evident that the X-bracing provide more stiffness for
the platform A compared to other bracing types. X-bracing scheme provides additional
load paths and redundancy to the substructure. At the angle of 90 degree, the collapse
load for all bracing types had similar peak but differed in magnitude. Lesser ultimate
loads are shown by single diagonal bracing where the framework created by a single
cross member provide less efficient load paths for load to be shared. The single diagonal
bracing had collapse loads close to the design basis but lower. This is due to the
presence of additional members in the top and bottom panel in the design basis. RSR
diagram (Figure 15) indicates that the X-bracing resulted in more strength reserve
compared to the other bracing schemes. The bracing provided more reserve strength
before the structure reached the critical point of ultimate load. The design basis had
performance in between the X-braced platform and the Single diagonal braced platform.
From Table 9 and Figures 14-15 the maximum and minimum performance of the
bracing schemes studied and corresponding wave directions are summarized for
platform A in Table 10.
From Table 10, it is seen that the platform has more stiffness for incoming wave at 90
degree. Higher loads are required to make the structure collapse from the 90 direction
than the other directions. All bracing configurations provide with the highest load at
wave direction of 90 degree which are parallel to individual base shear generated. So it
makes sense to orient the platform’s strongest direction with the strongest wave load
direction.
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
149
Interpreting the tabulated data above, the single diagonal is the weakest, X-bracing
provide highest rigidity while the original or design basis is in between the X bracing
and the diagonal bracing. The circumstances indicate that the design for platform A is
adequate and effective to the environmental area of the site location. As for conclusion,
the original design of platform A is cost effective and suitable with the surrounding area.
Table 10: Summary of bracing configuration study results
Bracing
Configuration
Design Basis
X-bracing
Single Diagonal
Bracing
Collapse Load (kN)
Minimum
(direction)
9018.19
(317.89º)
9144.72
(42.11º)
9029.29
(42.11º)
Maximum
(direction)
12974.80
(90.00º)
15280.47
(90.00º)
11221.38
(90.00º)
RSR
Minimum
(direction)
1.03
(0.00º)
1.02
(180.00º)
1.01
(270.00º)
Maximum
(direction)
1.20
(222.11º)
1.35
(222.11º)
1.20
(317.89º)
Due to leg arrangement of platform A at ROW B, the other types of bracing like the Kbracing, the Inverted K-bracing and the Diamond bracing are inappropriate. The
problem occurs where the Launch Cradle is used for sliding the jacket onto the barge
used for installing the structure. With the launch cradle attached to the substructure,
there was no horizontal member framing at designed elevation. The horizontal was
offset by several dimensions to cater the launch cradle framing. With one face of the
jacket structure not suitable for the other types of bracing, they were not chosen. The
strength of the original jacket would be affected by adding horizontal member for the K
member to intersect.
6.0
Conclusions and Recommendations
Platforms beyond design life need an assessment for the ultimate capacity for further
service. A study was carried out to check the platform reliability for extended service.
The first part of analysis clearly concluded that larger number of legs affect the overall
strength. Platform C an eight-legged platform gave the highest ultimate load compared
to the 4-legged platform and tripod. The Reserve Strength Ratio (RSR) of the platforms
A, B and C ranged from 1.0 to 1.6 while the collapse load factor range from 2.0 to 9.69.
The highest ultimate load among the platforms studied was 38437.97 kN reported for
platform C at wave angle of 115.00º. The highest RSR among the platforms was for the
platform B was computed as 1.56 at 120.00º.
150
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
Platforms A, B and C represent the major and common platforms installed in the
country. The location of each platform at different oil blocks in the region was to mainly
check the environment factor effect on the structures. The location of platform A and C
in PMO and platform B in SKO are sufficient to cater to the differences between the two
environment conditions.
The platform A was modified with different bracing schemes. The highest collapse load
was achieved with X-bracing model at wave attack angle of 90 degree as 15280.47 kN.
RSR computed at the load was 1.32. For platform A bracing schemes, the RSR ranged
from 1.01 to 1.32 while the collapse load factor ranged from 2.18 to 5.76. The bracing
type K-bracing, Inverted K-bracing and Diamond bracing were excluded from the study
due to no horizontal member at specified elevation due to the installed launch cradle for
transportation and lifting procedure
Recommendations for the future studies include collapse analysis for platform designed
for API RP2A-LRFD and comparison with platform designed as per API RP2A WSD.
With different code, the results are expected to be similar but slightly different in
behaviour of the load paths and method of computation. In the API RP2A-LRFD
approach to solution load factors are used in computing and there are also differences in
the value of constants such as drag, inertia and others. Comparison of the results
obtained with the LRFD and WSD code can help us to differentiate one code from the
other. Furthermore, the trend of higher utilization in the LRFD code can be in analyzed
using the collapse load. The comparison provides better accuracy and also enables cost
effective decisions for maintenance.
Alternate method of analysis of the models for Linear Static Analysis is the Simplified
Ultimate Strength (SUS) Analysis. This analysis comprises of the same procedure by
incrementing the load combination of storm wave until one the component fail or meets
capacity namely joints, members, pile steel strength , and pile soil bearing capacity. The
analysis is further done by removing the failed component to allow the alternative load
paths which exist in the framework. The analysis is completed when the software cannot
find solution meaning that no more load paths are available or exceed deformation. The
corresponding load is the collapse load and the base shear generated by the directional
waves.
In order to have better comparison, all types of conventional bracing should be
evaluated including the K-bracing, Inverted K-bracing and Diamond bracing which
were excluded in this analysis. For this, platforms which are appropriate for the bracings
mentioned should be chosen. A study of three or eight legged platform can provide data
on how bracing configuration affect such a configuration. Platform B and C can be
taken for such a bracing configuration studies. The overall data can provide a
comprehensive idea on the performance of different bracing schemes.
Malaysian Journal of Civil Engineering 25(2): 128-153(2013)
7.0
151
Acknowledgements
The authors acknowledge the use of the SACS program available at UTP as also the
data on the three platforms provided for which the source is not revealed.
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